1. Field of the Invention
The present invention is concerned with handover of a call between mobile communication networks. The present invention is particularly concerned with handover between UMTS networks and GSM networks but is also applicable to handover between different GSM networks and different cellular mobile communication networks generally.
2. Description of the Related Art
In current GSM systems, it is possible for a user to select manually among a plurality of available GSM networks when a mobile device (User Equipment) is off line. It is also possible for the User Equipment to select a network automatically from among the available networks based on preference stored in the User Equipment, normally in the SIM card. However, when a call is in progress, this selection is not possible. In the present specification, the term User Equipment is intended to encompass any equipment capable of communicating with a network; this will typically be a mobile telephone, but may be, for example, a dedicated data, facsimile, e-mail or video communication device or combination device.
During call progress, the majority of the User Equipment's radio resource are involved in the active call and so it is not possible to scan broadly for available networks. To enable a mobile device to move between cells, while a call is in progress, the network supplies a limited list of adjacent cells, normally confined to cells provided by the same network provider and permits the mobile device to make limited investigation of signal strength from the neighboring cells. In the event that the mobile device finds a stronger signal from another cell, it can signal a change to the other cell.
According to above prior method, it is not impossible to hand over between mobile communication networks while call is in progress.
It has been appreciated that it would be desirable to enable User Equipment to select a preferred one of a plurality of available networks while a call is in progress. This would be particularly beneficial when handing over from a UMTS network to a selected GSM network’, but would also be useful, particularly when traveling outside the User Equipment's home network territory, to enable handover between preferred “foreign” networks.
It has been appreciated that, although this is not normally done, it would be possible for an active network to supply a list of cells not only belonging to the active network, but also including cells of other network providers. This potentially offers a solution to the problem mentioned above, and may be provided independently in an aspect of the invention.
However, to implement the system, the network provider would have to supply a significant amount of information concerning other network providers' cells. Therefore, the above mentioned potential solution has the attendant problem that the list of available cells may become too large in practical terms for the User Equipment to investigate during an active call, and this may degrade performance of the network. This solution may therefore be undesirable for many network providers.